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Transit of Venus

TRANSITS OF VENUS are rare astronomical alignments in which the planet Venus crosses the face of the Sun as seen from Earth. They occur in pairs (eight years apart) separated at intervals of 105.5 and 121.5 years.

In 1717, astronomer Edmond Halley explained how simultaneous observations of a transit from far flung places could be used to calculate the distance from the Earth to the Sun. He urged astronomers to collaborate in order to determine the dimensions of the solar system–one of the great unsolved problems of the day!

In response, many astronomers sailed to remote parts of the world in order to observe the transits of 1761 and 1769. Harvard professor John Winthrop observed the first from St. John's, Newfoundland, and the second from Cambridge with the instruments shown here.

Research behind Enemy Lines

In 1761, John Winthrop was the only observer of the Transit of Venus in North America. The event was not visible from Cambridge, so Winthrop sailed to Newfoundland, taking two students and College apparatus behind enemy lines during the French and Indian War. The General Court of Massachusetts Bay financed the expedition and gave Winthrop letters of safe passage.

When Benjamin Franklin urged John Winthrop to go to Lake Superior in 1769 to observe the Transit of Venus, Winthrop had a problem–no money! The General Court, which had funded Winthrop's transit expedition in 1761, did not exist in 1769. It had been dissolved by the British governor for its support of patriot, Samuel Adams and his protest against taxation without representation.

So Winthrop sought money from General Thomas Gage, the British commander-in-chief, but Gage was too busy moving warships and troops into Boston in order to keep the peace. Gage offered wilderness guides but no funding.

When the expedition collapsed, Winthrop decided to observe the Transit of Venus from Harvard Yard. He used new instruments acquired in London with Franklin's help.